298 



coronoid process is more developed, and the great oval hole, 

 so characteristic of the crocodile's lower jaw, and probably 

 serving for attachment to the masseter, is reduced to a long 

 deep sinus. In these respects the ichthyosaurus resembles 

 the monitor and other lacertians, but differs in the general 

 contour of all this part and of its constitution bones, especi- 

 ally in the most entire concealment of the articular bone 

 within it, which, in these, forms all the external and poste- 

 rior part ; the ichthyosaurus agrees in these points with the 

 crocodile. 



In the head the position of the nostrils is peculiar, being 

 behind the snout, and close in front of the orbits ; but the 

 posterior opening of the nasal canal into the throat is placed 

 far back, behind the internal pterygoid processes, which, as 

 in the crocodile, form a continuous plate beneath it ; conse- 

 quently the whole structure of the roof of the mouth 

 assumes an arrangement completely agreeing Avith the 

 crocodile, and entirely distinct from that of the other lacer- 

 tians. The bones surrounding the orbit are not very distinc- 

 tive, but agree rather with the crocodile than with other 

 lacertae ; the sclerotica, however, resembles that of the 

 latter class, by its division into scales. 



The cavities behind the orbits, compared in the Geo- 

 logical Transactions to those similarly placed in the crocodile, 

 bear, in their extent, a greater analogy to the temporal 

 fossae of other lacertae, since the temporal muscle which is 

 attached near their edges, being in both the ichthyosaurus 

 and lacertians the principal mover of the lower jaw, requires 

 greater space, whereas, in the crocodile, it is scarcely called 

 into play, except in the gavial, where these fossae neces- 

 sarily have their dimensions much enlarged. These post- 

 orbital fossae of the crocodiles, however, may be considered 

 as the true analogues of the temporal fossae in the other 

 lacertae, serving the same purposes and being surrounded 

 by the same bones, so that the difference in this respect is 



